Abstract

An experiment was conducted to characterise population dynamics of lice and associated production loss in extensively grazed flocks infested at different times after shearing. Merino wethers were allocated to six groups of 31 sheep. In the first year (Y1), one sheep in each of two groups was infested with 2500 lice at 6 weeks after shearing (September), sheep in two groups were infested at 4 months (December) and two groups remained uninfested. In year 2 (Y2), infestations carried over from Y1, but a lousy sheep was added to each 6-week infestation mob. In year 3 (Y3), the infested mobs were treated and remained lice free, while lousy donor sheep were added to the two previously uninfested mobs. In Y1, lice appeared to die out in one 6-week-infestation group and were found on only two sheep before next shearing in the other. Lice persisted and spread in the two 4-month-infestation flocks, reaching mean counts (±s.e.) of 0.5 (±0.2) and 0.2 (±0.1) per part before shearing. In Y2 and Y3, lice persisted and increased in all infested flocks, reaching mean counts of 2.3 (±0.6), 8.5 (±1.5), 3.6 (±0.6) and 2.8 (±0.7) per part in Y2 and 1.0 (±0.2) and 1.2 (±0.4) per part in Y3. The count of 8.5 was in the flock with both a carry-over infestation and an infestor sheep. Exponential and logistic models were fitted to describe lice increase; differences in fleece derangement reflected louse numbers. Clean fleece weight was higher in flocks without lice in all years (0.12 kg/head in Y1; 0.22 kg/head in Y2 and Y3; P < 0.05). Classer-assigned colour scores (although not measured colour), cott score and line into which the wool was classed also differed significantly (P < 0.05) between infested and non-infested flocks and there was an indication that staple length was reduced in more heavily infested flocks. In spring-shorn flocks in environments with high solar radiation and no lice present at shearing, even if lice subsequently enter the mob, it appears unlikely that they will increase to levels where serious economic loss will be experienced before next shearing. The study also indicated that lice could persist in flocks at levels unlikely to be detected by most commercial wool producers for extended periods, possibly through one season, which may help to explain reports of new infestations with no apparent source.